Project Summary: Sound localization ? determining where in space a sound originates ? is a cardinal task of the auditory system. While much progress has been made in uncovering the fundamental neural processes that mediate sound localization, there still remain significant gaps that limit our understanding of the underlying circuit. Here we focus on the role played by one key component of the sound localization circuit, the medial nucleus of the trapezoid body (MNTB), and its inhibitory output. The overarching hypothesis of this proposal is that fast MNTB inhibition is essential for the neural computation of ITDs and IIDs at the two sound main localization nuclei, the medial and lateral superior olive (MSO and LSO, respectively). In addition, we propose that MNTB inhibition will have a sufficiently large effect on the generation of wave III of auditory brainstem responses (ABRs), such that suppression of MNTB will alter this wave. ABRs are a method that non-invasively measure in-vivo the activity of brain stem neurons, and we focus on wave III because in rodents this wave is commonly associated with activity of neurons in the sound localization nuclei. To test the hypothesis, we will optogenetically manipulate MNTB and record MSO activity to discriminate between several competing models of sound localization by MSO neurons (Aim 1). Then we will optogenetically manipulate MNTB and record low-frequency LSO activity to study a population of low-frequency LSO neurons (Aim 2). Finally, we will optogenetically suppress MNTB and record responses and effects on the ABR (Aim 3). This manipulation mimics similar ABR alterations in common hearing conditions affecting binaural hearing. Overall, the aims are designed to enhance our understanding of sound localization mechanisms in the auditory brainstem, the brain area that performs the first separation of multiple sounds from each other based on their spatial location. There are a number of medical conditions in which alterations in this pathway lead to a patient's decreased ability to localize sound and function in a noisy environment. A better understanding of the neural mechanisms in this circuit in the healthy auditory system will help design treatments for these conditions in the future.